The present invention relates to glass fiber optical wave guides useful in a laser communication or data transmission system.
The most promising transmitting media for optical communication signals is the glass fiber optical wave guide. In an optical wave guide the light beam is confined within and guided along a solid glass fiber of cylindrical cross section with a plurality of fibers assembled into a bundle and protected with a sheathing so that the wave guide can be handled much like a cable. The individual fibers are generally formed of a central glass core surrounded by a glass cladding which has a lower refractive index than the core so that a light beam injected into one end of the compound fiber is confined to and guided along the core by total internal reflection. The fiber cores range in diameter from approximately 1 to 100 micrometers and typical cladding-core diameter ratios are on the order of 1.1 to 1.
Another type of glass fiber optical wave guide, which is referred to as a self-focusing fiber, has a radially parabolic refractive index profile created within the glass fiber. The variation in the refractive index is intended to cause the light beam to be continuously focused or guided along the filament. The desired refractive index profile is sought to be achieved by ion exchange techniques in which, for example, thallium ions in the glass fiber near the surface are replaced by sodium or potassium ions. In the latter type of wave guide, it is quite difficult to achieve the desired distribution of the ions, and unduly long ion exchange times are required when following prior art ion exchange procedures.
The preferred glass compositions for optical fibers are generally silicates. The raw materials from which the glass compositions are obtained can be in various forms such as oxides and carbonates, but must be of high purity. Specifically, transition metals such as ferrous ion, nickel and chromium should be essentially completely excluded, and preferably are present in an amount of less than 2 parts per billion.
In the production of the fibers, a melt is prepared and the fiber is drawn either using the standard double crucible technique used for pulling fibers, or from preforms made of tubes and rods of appropriate dimensions to be used in the preform technique for preparing glass fibers (e.g., A. D. Pearson and A. R. Tynes, "Light Guidance in Glass Media." American Ceramic Soc. Bull. 49, 969-973 (1970).
When preparing a self-focusing fiber, the drawn fiber may be passed directly into a molten salt bath for the ion exchange treatment.
At the present time, an important problem to be solved in adapting glass fiber optical waveguides for use as a transmission line in communications systems which extend over long distances is the need to produce low signal loss silicate fibers by relatively inexpensive commercial production processes. One of the problems faced is to reduce loss due to extrinsic absorption which is absorption due to the presence of molecular or ionic impurities, especially Fe.sup.+.sup.+ ions and other transition elements. Production processes seeking to alleviate this problem are made much more expensive and complicated by efforts to keep out or remove the iron impurities from the silicate glasses. The iron gets into the glass both from the raw materials and in the melting procedure.
The methods proposed thus far to avoid this problem can be quite costly. An example of this is the vapor deposition technique. (See Keck et al, U.S. Pat. No. 3,711,262 on vapor deposition.)